Citizens help a lady who fainted while attending a rally in protest against a shelling by Ukrainian government forces in the village of Semyonovka, Ukraine, on May 22. / Alexander Zemlianichenko, AP

by Portia Walker, Special for USA TODAY

by Portia Walker, Special for USA TODAY

DONETSK, Ukraine - As Ukraine prepares for a crucial presidential election, 16 soldiers and a pro-Russian rebel were killed Thursday in the deadliest attack on the military since unrest began in the east of the country in April.

The Ukrainian soldiers were killed at a checkpoint in the village of Blahodatne, 20 miles from Donetsk, after dawn clashes with pro-Russian separatists. One of the rebels was killed in the raid, and more than 30 soldiers were injured, the Associated Press reported.

Sunday, Ukrainians will elect a president to replace Oleksander Turchynov, who has led the country since anti-government protesters ousted President Victor Yanukovych in February.

The front-runner is Victor Poroshenko, a billionaire confectionary tycoon, dubbed the "chocolate king." He's promised to keep Ukraine together but grant more autonomy to restive pro-Russian regions in the east.

Trailing behind him in polls is former prime minister Yulia Tymoshenko, an outspoken opponent of Russian President Vladimir Putin. Tymoshenko has promised to reduce Russian influence in the country.

If none of the candidates wins 50% of the vote, there will be a runoff between the two leading contenders June 8.

Thirty-six million people are eligible to vote in the election, but a fifth of them live in areas affected by pro-Russian unrest. There will be no elections in Crimea, which was annexed by Russia in March.

Fears that the election will be disrupted in the east have mounted after raids on polling stations by gunmen who have threatened election officials.

Many preparations for the poll are conducted in secret to prevent attacks. Sergei Trachenko, head of the Donetsk branch of the Committee of Voters of Ukraine, an electoral monitoring body, would not confirm whether ballots and electoral lists had arrived in the region. He said election day posed many challenges. "We cannot say how many polling stations will be open. Perhaps it will be 50% of them, but right now, nobody knows," he said.

Peaceful elections could mark a turning point in Ukraine's crisis, which erupted in late winter over divisions about whether the former Soviet republic, which has a larger ethnic Russian population in the east, should forge closer ties with Western Europe or remain close to neighboring Russia.

Separatist leaders in the self-declared Donetsk People's Republic have told voters to boycott the election. They claim that the territory no longer belongs to Ukraine after they declared independence following a referendum May 11, when voters overwhelmingly supported self-determination. Ukraine's central government and Western governments have called the referendum invalid.

Russia has said the Kiev government is illegitimate; however, Putin has recently softened his position on Sunday's elections, calling them a "step in the right direction."

Putin said Russia is pulling back troops from its border with Ukraine, where NATO estimates it has 40,000 stationed.

Limited Russian troop activity along the Ukrainian border "may suggest that some of these forces are preparing to withdraw," NATO Secretary-General Anders Rasmussen said on his Twitter account. But he warned, "Most of previously deployed Russian force remains near the Ukrainian border. We see continued Russian exercises."